[openssl/openssl] 7d2680: ssl/tls_srp.c: Add check for BN_dup

[Todd Short]

  Branch: refs/heads/openssl-3.0
  Home:   https://github.openssl.org/openssl/openssl
  Commit: 7d268078e2178b4bfa7e686c1dd4e5688a07604a
      https://github.openssl.org/openssl/openssl/commit/7d268078e2178b4bfa7e686c1dd4e5688a07604a
  Author: Jiasheng Jiang <jiasheng at iscas.ac.cn>
  Date:   2022-07-08 (Fri, 08 Jul 2022)

  Changed paths:
    M ssl/tls_srp.c

  Log Message:
  -----------
  ssl/tls_srp.c: Add check for BN_dup

As the potential failure of the BN_dup,
it should be better to check the return value
in order to guarantee the success.

Signed-off-by: Jiasheng Jiang <jiasheng at iscas.ac.cn>

Reviewed-by: Tomas Mraz <tomas at openssl.org>
Reviewed-by: Shane Lontis <shane.lontis at oracle.com>
Reviewed-by: Todd Short <todd.short at me.com>
(Merged from https://github.com/openssl/openssl/pull/18699)

(cherry picked from commit 12e488367d34657a5c0e1bc322e66c48463d2a0c)